Permanent mold casting is a metal casting process that employs reusable “permanent” metal molds. It is often referred to as gravity die casting because metal is typically poured into the die under gravity-induced pressure instead of using forcible injection. Some manufacturers use other processes as well, such as pouring molten metal under high gas pressure or vacuum pressure.
Permanent mold casting is used primarily for high-volume production of smaller to mid-size metal parts that are required to have uniform wall thickness. They can be manufactured out of magnesium, aluminum, copper and many metal alloys, and weigh anywhere from 1.5 ounces to 150 lbs. Since permanent mold casting has only a moderate ability to produce intricate details, it is best used for structurally simpler parts.
Examples of metal parts frequently created in large production runs using permanent mold casting include: pipe fittings, cylinder blocks, cylinder heads, gear blanks and housings, connecting rods, and automotive components such as wheels, pistons and impellers.
Permanent mold casting uses a strong metal mold, typically one made from steel, cast iron or a dense cast iron trademarked as Meehanite. The cost of producing the mold can be significant, but since it is permanent, the mold can be reused thousands of times. This combination of long mold life and large product volumes that can be manufactured with the same mold offsets mold production costs and makes the process very economical.
Permanent mold casting is generally used for shaping non-ferrous metals that include aluminum, magnesium, and copper alloys. However, graphite molds can also be used to cast iron, steel and some other metals.
A permanent core of metal or an expendable core of sand is often inserted inside the permanent mold before pouring in the molten material and clamping the mold shut. Molten material is then allowed to cool and solidify into the desired part shape within the mold to designated temperatures.
After opening the mold, the part is taken out and finishing work is performed. This removes excess castings from the gate area and attached minor imperfections and flashings along places where sections of the mold and/or core joined together.
Step-by- step process description
Permanent mold casting typically involves six steps:
1. Custom mold creation
The first step of permanent mold casting is creating a custom mold. Each component of the mold is carefully machined from individual metal blocks, making sure that they fit together precisely, and open and close easily and accurately. The mold must also include a gate area to ensure that molten materials can enter safely.
Once the mold is created, it can be used thousands of times before its mold life expires. Mold life is defined as the number of castings produced by a mold over its useful lifespan. This property typically depends upon several factors, including: mold materials, casting materials and operating temperatures.
2. Mold preparation
Typically, before casting, the mold is preheated to between 300 degrees Fahrenheit and 500 degrees Fahrenheit. Heated molds permit smoother metal flow and reduce defects. Preheating also reduces the possibility of damage to the mold itself from hot molten metals.
A thin protective layer of clay or sodium silicate may also be applied to interior mold cavities to function as a thermal gradient and lubricant for easier removal of the cast part, and to additionally protect the mold from damage.
3. Mold assembly
Permanent casting molds typically consist of at least two parts, including the two mold sections and any cores used to form complex features. All the parts need to be closed and held together with force prior to the pouring of molten metal inside. This is normally done with the help of machinery.
Molten material enters the mold from the top at a slow rate through a gate area accessed by an opening called a sprue.
5. Cooling and mold opening
Molten metal is left to slowly solidify within the mold and cool to a desired temperature. Since many metals somewhat contract in size during the cooling process, solid metal casting needs to be removed from the mold before it cools completely to prevent the mold from cracking. This is done by means of ejector pins that are built into the mold.
The cast metal part is ready for the final removal of any excess metal fragments.
Characteristics of permanent mold castings
Permanent mold casting produces reduced levels of trapped gas bubbles inside metal parts while the nature of the mold allows the casting to solidify quickly and with only minor shrinkage. As a result, cast products have smaller metal grain structure, greater uniformity in every part of the cast part, and superior mechanical properties.
Permanent mold castings are also characterized by a good surface finish. Defects such as traces of metal flow lines are only cosmetic in nature, rather than functional. They can be easily removed during the finishing phase of the casting process.